1. Field of the Invention
This invention relates to a method of economically producing non-oxide ceramic sintered bodies of high quality by subjecting a non-oxide ceramic shaped body to hydrostatic pressing treatment and/or machining without damaging and then sintering. Particularly, it is concerned with the production of silicon nitride articles having a complicated shape such as a turbocharger rotor, sub-combustion chamber of a diesel engine, and the like.
2. Related Art Statement
Heretofore, it was very difficult to machine such ceramic articles after sintering. Thus it was important to reduce the amount of machining necessary after the sintering by previously shaping the ceramic body into a shape as close as possible to that of the final product after the sintering, thus reducing the time and expense required to manufacture the article.
In general, when applying a shaping method capable of forming only a relatively simple shape such as mold pressing or the like, the solution of the above problem is achieved by subjecting the shaped body to a machining before the sintering. However, the shaped body before the sintering is very brittle, so that it is necessary to calcine the body before machining to increase the strength of the shaped body.
As non-oxide ceramics, nitrides such as silicon nitride, aluminum nitride and the like are calcined in a nitrogen gas atmosphere for preventing decomposition of the starting material. However, in the conventional method, the strength of the shaped body is not sufficiently increased, and consequently there is a problem of chipping and the like during the machining.
Further, when using shaping methods such as injection molding, slurry casting or the like, which are capable of forming a complicated shape, a shaped body requiring no machining after the shaping step can be obtained. However, it is desired to further increase the density of the shaped body for obtaining a higher density ceramic body. In this connection, Japanese Patent laid open No. 55-20259 discloses a method of producing a high density ceramic body wherein the shaped body obtained by the injection molding method is covered with rubber latex, subjected to a hydrostatic pressing treatment and then sintered. This technique has a drawback that since the shrinkage of the shaped body is different between the surface and the inside in the hydrostatic pressing, strain is caused after the hydrostatic pressing to thereby injure the hydrostatic pressed body. Therefore, it is considered that the shaped body can not finally overcome the generated strain because the strength is low.
Moreover, a method of raising the calcining temperature is generally known as the method of increasing the strength of the shaped body. However, as the calcining temperature becomes higher, the strength of the calcined body rapidly increases (i.e., approaches that of a sintered body); so that it is very difficult to control the calcining temperature for obtaining the desired strength.